Glucagon is a 29-residue peptide hormone that regulates glycogenolysis and glucogenesis. The structure of glucagon may be represented as follows: ##STR1##
The abbreviations utilized herein are those recommended by IUPAC-IUB [see Eur. J. Biochem. 138, 9 (1984)].
The nomenclature used to define the peptides is that specified by Schroder & Lubke, "The Peptides", Academic Press (1965), wherein in accordance with conventional representation the amino group at the N-terminal appears to the left and the carboxyl group at the C-terminal to the right. Where the amino acid residue has isomeric forms, it is the L- or naturally occurring form of the amino acid that is represented unless otherwise expressly indicated.
Insulin, as is known, rapidly decreases elevated blood sugar.
It is believed that, in humans, diabetes is only observed when insulin levels are low and glucagon levels are simultaneously elevated. The absence of insulin allows blood glucose to rise particularly after a meal, and the presence of glucagon causes a further rise in blood glucose. Large amounts of insulin are required to reduce the glucose levels to normal. The maintenance of stable levels is difficult and subject to considerable fluctuation. This wide fluctuation is responsible, at least in part, for the clinical difficulties experienced in diabetes.
Glucagon appears to act by binding to liver membrane receptors thereby activating adenylate cyclase which, in turn, triggers a series of reactions including the production of cyclic adenosine monophosphate (cAMP), which activates phosphorylase and inhibits glycogen synthetase, thereby contributing to elevated glucose levels in the blood.
Recently considerable effort has been expended to develop glucagon antagonists that will bind to the liver membrane but do not have the ability to transduce the signal to activate adenylate cyclase. One such product is N.alpha.- trinitrophenyl [12-homoarginine] glucagon. This product does bind to the glucagon receptor without significant activation of adenylate cyclase. Unfortunately it activates another binding system in the hepatocyte membrane leading to the production of inositol trisphosphate and calcium ions. A useful antagonist will block the action of endogenous glucagon by preventing it from binding to the liver membrane receptors and thereby producing cAMP and glucose in the cell, and the ultimate elevation of blood sugar. Such products would be useful to reduce a diabetic's need for injections or infusion of insulin.
An ideal glucagon antagonist would (1) be completely inactive toward stimulation of adenylate cyclase and production of cAMP, (2) bind as well as glucagon itself to the liver membrane, (3) compete with glucagon for binding to the membrane, (4) at moderate concentrations fully inhibit the action of glucagon toward the activation of adenylate cyclase, and (5) have a satisfactory inhibition index.
The inhibition index is the molar ratio of antagonist to agonist which reduces the biological response to one half of the value for the agonist in the absence of antagonist. It will be discussed more fully hereinafter.